Small molecules that control the expression and/or activity of transgene products can be used to initiate signaling pathways that regulate cellular functions, including cell death. Studies supported by this grant have investigated inducible suicide genes based on the induction of apoptosis through the chemical induced dimerization of FK506 binding proteins (FKBP) linked to death effector molecules. This strategy for regulating cell survival employs transgenes that encode human proteins and should avoid the problem of immune responses to suicide gene products derived from pathogens. A new construct that utilizes an inducible caspase-9 gene (iCasp-9) for cell suicide and a truncated CD34 molecule for selection of transduced cells has been developed to overcome the limitations of the Fas suicide gene studied in the prior funding period. The proposed studies will evaluate retroviral vectors encoding the iCasp-9 suicide gene in vitro and in a clinical trial to regulate allogeneic donor T lymphocytes administered to patients to treat recurrent or persistent malignancy after allogeneic hematopoietic stem cell transplant (HSCT). The introduction of iCasp-9 will bring the T cells under pharmacologic control and permit their ablation in patients with GVHD. These studies will provide insight into the clinical use of chemical dimerizing agents to regulate transgene function and may identify a suicide gene that is not immunogenic in humans and could be broadly applied in cell and gene therapy. The specific aims are: 1. To determine the in vitro activity of retroviral constructs encoding truncated CD34 as a selectable marker and inducible caspase-9 (iCasp-9) for cell suicide. 2. To determine the safety, in vivo persistence, and biologic activity of adoptive immunotherapy with donor T lymphocytes modified by retrovirus mediated gene transfer to express the ACD34-iCasp-9 suicide gene. 3. To determine if donor T lymphocytes modified to express the ACD34/iCasp-9 suicide gene can be ablated in patients who develop graft versus host disease or other serious toxicity by the administration of the synthetic FKBP binding drug AP1903.